In a staged fracturing operation, multiple zones of a formation need to be isolated sequentially for treatment. To achieve this, operators install a fracturing assembly downhole in the wellbore, which typically has a top liner packer, open hole packers isolating the wellbore into zones, various sliding sleeves, and a wellbore isolation valve. When the zones do not need to be closed after opening, operators may use single shot sliding sleeves for the fracturing treatment. These types of sleeves are usually ball-actuated and lock open once actuated. Another type of sleeve is also ball-actuated, but can be shifted closed after opening.
Initially, operators run the fracturing assembly in the wellbore with all of the sliding sleeves closed and with the wellbore isolation valve open. Operators then deploy a setting ball to close the wellbore isolation valve. This seals off the tubing string of the assembly so the packers can be hydraulically set. At this point, operators rig up fracturing surface equipment and pump fluid down the wellbore to open a pressure actuated sleeve so a first zone can be treated.
As the operation continues, operates drop successively larger balls down the tubing string and pump fluid to treat the separate zones in stages. When a dropped ball meets its matching seat in a sliding sleeve, the pumped fluid forced against the seated ball shifts the sleeve open. In turn, the seated ball diverts the pumped fluid into the adjacent zone and prevents the fluid from passing to lower zones. By dropping successively increasing sized balls to actuate corresponding sleeves, operators can accurately treat each zone up the wellbore.
FIG. 1A shows an example of a sliding sleeve 10 for a multi-zone fracturing system in partial cross-section in an opened state. This sliding sleeve 10 is similar to Weatherford's ZoneSelect MultiShift fracturing sliding sleeve and can be placed between isolation packers in a multi-zone completion. The sliding sleeve 10 includes a housing 20 defining a bore 25 and having upper and lower subs 22 and 24. An inner sleeve or insert 30 can be moved within the housing's bore 25 to open or close fluid flow through the housing's flow ports 26 based on the inner sleeve 30's position.
When initially run downhole, the inner sleeve 30 positions in the housing 20 in a closed state. A breakable retainer 38 initially holds the inner sleeve 30 toward the upper sub 22, and a locking ring or dog 36 on the sleeve 30 fits into an annular slot within the housing 20. Outer seals on the inner sleeve 30 engage the housing 20's inner wall above and below the flow ports 26 to seal them off.
The inner sleeve 30 defines a bore 35 having a seat 40 fixed therein. When an appropriately sized ball B lands on the seat 40, the sliding sleeve 10 can be opened when tubing pressure is applied against the seated ball B to move the inner sleeve 30 open. To open the sliding sleeve 10 in a fracturing operation once the appropriate amount of proppant has been pumped into a lower formation's zone, for example, operators drop an appropriately sized ball B downhole and pump the ball B until it reaches the landing seat 40 disposed in the inner sleeve 30.
Because the zones of a formation are treated in stages with the sliding sleeves 10, the lowermost sliding sleeve 10 has a ball seat 40 for the smallest ball size, and successively higher sleeves 10 have larger seats 40 for larger balls B. In this way, a specific sized ball B dropped in the tubing string will pass though the seats 40 of upper sleeves 10 and only locate and seal at a desired seat 40 in the tubing string.
Once the ball B is seated, built up pressure forces against the inner sleeve 30 in the housing 20, shearing the breakable retainer 38 and freeing the lock ring or dog 36 from the housing's annular slot so the inner sleeve 30 can slide downward. As it slides, the inner sleeve 30 uncovers the flow ports 26 so flow can be diverted to the surrounding formation. The shear values required to open the sliding sleeves 10 can range generally from 1,000 to 4,000 psi (6.9 to 27.6 MPa).
Once the sleeve 10 is open, operators can then pump proppant at high pressure down the tubing string to the open sleeve 10. The proppant and high pressure fluid flows out of the open flow ports 26 as the seated ball B prevents fluid and proppant from communicating further down the tubing string. The pressures used in the fracturing operation can reach as high as 15,000-psi.
After the fracturing job, the well is typically flowed clean, and the ball B is floated to the surface. Then, the ball seat 40 (and the ball B if remaining) is milled out. The ball seat 40 can be constructed from cast iron to facilitate milling, and the ball B can be composed of aluminum or a non-metallic material, such as a composite. Once milling is complete, the inner sleeve 30 can be closed or opened with a standard “B” shifting tool on the tool profiles 32 and 34 in the inner sleeve 30 so the sliding sleeve 10 can then function like any conventional sliding sleeve shifting with a “B” tool. The ability to selectively open and close the sliding sleeve 10 enables operators to isolate the particular section of the assembly.
The standard fracture sleeve system as noted above uses graduated ball seats that are permanently installed into the sleeves during the fracturing operation. Once all of the sleeves have been opened, reservoir pressure is then relied upon to pump the balls off the seats so the balls can then float and be carried by flow back to surface. In the case where a ball fails to pump off the seat or if a larger flow area is desired, operators run an intervention to mill out the ball and seats to remove the obstructions.
To facilitate milling, anti-rotation features have been added to the inserts in the sliding sleeves in both the opened and closed position so that the ball and seat can be milled out in any orientation. Also, dissolvable ball materials have been used to eliminate the potential need for milling.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.